The invention concerns a measuring cell for a gas analysis spectrometer with an inner chamber for a gas to be analyzed, an inlet and an outlet, wherein a traversing optical path for a measuring beam is formed in the inner chamber.
The optical analysis of gases is applied widely in various areas of technology. Special requirements are demanded by exhaust gas measurement applications for internal-combustion engines. Due to increasingly strict exhaust gas regulations, not only is a high level of sensitivity required to achieve a low detection threshold, but also a high time resolution to ensure a sufficiently good dynamic response of the measurement, in particular, with respect to non-stationary operating states of internal-combustion engines. This results in a conflict of objectives between detection sensitivity and the time resolution of the system. In such measuring cells for optical gas analysis devices, the detection sensitivity depends on the optical path length that the measuring beam travels through the gas to be analyzed in the measuring cell. This path length, in turn, depends on the inner chamber volume of the measuring cell and the guidance of the measuring beam. However, the time resolution that is decisive for the dynamic response directly depends on the time needed to replace the gas to be analyzed in the measuring cell. It is important that the gas is replaced in its entirety. Increasing the volume of the measuring cell therefore has the disadvantage that, while other parameters remain constant, the time required to completely replace the gas increases, causing the time resolution and therefore the dynamic response to decrease correspondingly.
Various approaches for increasing the quality of the measurement are known from prior art. In many measuring cells, attempts are made to increase the detection sensitivity for a constant cell volume by optimizing the optical path. U.S. Pat. No. 5,440,143 A1 describes attaching a special mirror system onto an otherwise standard measuring cell with a square cross-section, which produces a multiply folded and therefore extended optical path for the measuring beam. Disposing multiple measuring cells one behind the other so that the measuring beam is first guided through a first measuring cell and then through another, is known from US 2007/0182965 AI. A universal measuring cell for adapting the length of the optical path is known from JP 10/062335 A, wherein the cell is constituted as two telescopic partial bodies.
An alternative approach has tried to influence the flow of sample gas within the measuring cell (DE 103 18 786 A). In such a measuring cell, however, relatively large “dead zones” are formed, which increase the exchange time and worsen the dynamic response. As FIG. 7 schematically shows, in a measuring cell (9) according to prior art, swirling (91) of sample gas in the measuring cell causes formation of dead zones in which molecules of the sample gas can dwell for a comparatively long time, preventing fast exchange. As the concentration of the supplied sample gas (90) changes, the previous concentration is still partly present so that the new concentration value can only be correctly determined once the gas in the dead zones has also been exchanged.
The resulting time delay causes carryover (concentration carryover), which in turn results in a long response time of the measuring cell and therefore of the entire measuring system.
The object of the invention is to create an improved measuring cell with a better dynamic response.